1. Field of the Invention
The present invention relates to a multimedia broadcast/multicast service (MBMS) and, more particularly, to a method for transmitting a control signal for transmission of MBMS data in a universal mobile telecommunications system (UMTS).
2. Discussion of the Related Art
A universal mobile telecommunications system (UMTS) is a third generation mobile communication system that has evolved from a standard known as Global System for Mobile communications (GSM). This standard is a European standard which aims to provide an improved mobile communication service based on a GSM core network and wideband code division multiple access (W-CDMA) technology.
FIG. 1 shows a network structure of a general UMTS.
As shown in FIG. 1, the UMTS is roughly divided into a terminal, a UTRAN and a core network.
The UTRAN includes one or more radio network sub-systems (RNS). Each RNS includes an RNC and one or more Node Bs managed by the RNCs.
Node Bs are managed by the RNCs, receive information sent by the physical layer of a terminal (e.g., mobile station, user equipment and/or subscriber unit) through an uplink, and transmit data to a terminal through a downlink. Node Bs, thus, operate as access points of the UTRAN for terminal.
The RNCs perform functions which include assigning and managing radio resources, and operate as an access point with respect to the core network.
A primary function of UTRAN is constructing and maintaining a radio access bearer (RAB) for a call connection between the terminal and the core network. The core network applies quality of service (QoS) requirements of end-to-end to the RAB, and accordingly, UTRAN can satisfy the QoS requirements of the end-to-end by constructing and maintaining the RAB.
The RAB service is divided into an Iu bearer service and a radio bearer service of a lower concept. The Iu bearer service handles reliable user data transmission between boundary nodes of UTRAN and the core network, while the radio bearer service handles reliable user data transmission between the terminal and UTRAN.
FIG. 2 illustrates a radio protocol between the terminal and UTRAN on the basis of the 3GPP wireless access network standards.
With reference to FIG. 2, the radio protocol is vertically formed of a physical layer, a data link layer and a network layer, and is horizontally divided into a user plane for transmitting data information and a control plane for transmitting a control signal.
The user plane is a region to which traffic information of a user such as voice or an IP packet is transmitted. The control plane is a region to which control information such as an interface of a network or maintenance and management of a call is transmitted.
In FIG. 2, protocol layers can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model well known in a communication system.
The first layer (PHY) provides an information transfer service to the upper layer by using various radio transfer techniques.
The first layer is connected to the MAC layer through a transport channel, and data is transferred between the MAC layer and the PHY layer through the transport channel.
Data is transmitted according to transmission time interval (TTI) through the transport channel. The physical channel transfers data by dividing it by the unit of certain time called a frame. In order to synchronize the transport channel between the UE and UTRAN, a connection frame number (CFN) is used. The CFN value has the range of 0˜255 in case of transport channels except for a paging channel (PCH). That is, CFN is repeatedly circulated by the period of 256 frames.
Besides the CFN, a system frame number (SFN) is also used to synchronize the physical channel. The SFN value has the range of 0˜4095 and repeated by the period of 4096 frames.
The MAC layer provides a re-allocation service of the MAC parameter for allocation and re-allocation of radio resources.
The MAC layer is connected to the radio link control (RLC) layer through a logical channel, and various logical channels are provided according to the kind of transmitted information. In general, when information of the control plane is transmitted, a control channel is used. When information of the user plane is transmitted, a traffic channel is used.
The MAC is classified into an MAC-b sublayer, an MAC-d sublayer and an MAC-c/sh sublayer according to types of managed transport channels. The MAC-b sublayer manages a BCH (Broadcast Channel) handling broadcast of system information, while the MAC-c/sh sublayer manages shared transport channel such as FACH (Forward Access Channel), DSCH (Downlink Shared Channel), or the like, shared with other terminals.
In UTRAN, the MAC-c/sh sublayer is positioned at a control RNC (CRNC) and manages channels shared by every terminal in a cell, so that one MAC-c/sh sublayer exists in each cell.
The MAC-d sublayer manages a DCH (Dedicated Channel), a dedicated transport channel for a specific terminal. Accordingly, the MAC-d sublayer is positioned at a serving RNC (SRNC) managing a corresponding terminal, and one MAC-d sublayer exists also at each terminal.
A radio link control (RLC) layer supports a reliable data transmission and may perform a function of segmentation and concatenation of an RLC service data unit (SDU) coming from a higher layer. The RLC SDU transferred from the higher layer is adjusted in its size according to a throughput capacity at the RLC layer, to which header information is added, and then transferred in a form of a PDU (Protocol Data Unit) to the MAC layer. The RLC layer includes an RLC buffer for storing the RLC SDU or the RLC PDU coming from the higher layer.
A broadcast/multicast control (BMC) layer performs functions of scheduling a cell broadcast message (CB) transferred from the core network and broadcasting the CB to UEs positioned in a specific cell(s). At the side of UTRAN, the CB message transferred from the upper layer is combined with information, such as a message ID, a serial number or a coding scheme, and transferred in a form of BMC message to the RLC layer and to the MAC layer through a CTCH (Common Traffic Channel), a logical channel. In this case, the logical channel CTCH is mapped to a FACH (Forward Access Channel), a transport channel, and an S-CCPCH (Secondary Common Control Physical Channel), a physical channel.
A packet data convergence protocol (PDCP) layer is an upper layer of the RLC layer, allowing data to be transmitted effectively on a radio interface with a relatively small bandwidth through a network protocol such as the IPv4 or the IPv6. For this purpose, the PDCP layer performs a function of reducing unnecessary control information, which is called a header compression, and in this respect, RFC2507 and RFC3095 (robust header compression: ROHC), a header compression technique defined by an Internet standardization group called an IETF (Internet Engineering Task Force), can be used. In these methods, because the only information requisite for the header part of a data is transmitted, control information is transmitted, so that an amount of data transmission can be reduced.
The RRC layer positioned in the lowest portion of the third layer (L3) is defined only in the control plane and controls the logical channels, the transport channels, and the physical channels in relation to the setup, the reconfiguration, and the release of the RBs. The RB signifies a service provided by the second layer for data transmission between the terminal and UTRAN, and setting up the RB means processes of stipulating the characteristics of a protocol layer and a channel, which are required for providing a specific service, and setting the respective detailed parameters and operation methods.
A broadcast of the system information will now be described.
The broadcast of the system information is one of major function of the RRC layer. The system information includes various information such as system information with which the terminal connects to a network or terminal's mobility support information and measuring information. The system information is transferred through a broadcast control channel (BCCH), a logical channel, and can use the BCH or the FACH as a transport channel.
In order to systematically transmit the system information, the RRC layer constructs a system information block (SIB) by grouping system information with similar characteristics. System information belonging to a different SIB have different characteristics in their transmission repetition period as well as in their content. Substantial system information is included in the SIB, while scheduling information for transmission of the SIB is called on a master information block (MIB) or on a scheduling block (SB).
The MIB, including reference information or scheduling information of SIBs broadcast in a cell, is transmitted regularly through the BCH so that the terminal can easily receive the system information.
Because the MIB includes reference information or scheduling information on one or two SBs and the SB includes additional scheduling information of SIBs, transmission scheduling information of each SIB can be obtained through the MIB and the SB.
The multimedia broadcast/multicast service (MBMS) will now be described.
The MBMS is a service for transmitting multimedia data such as audio, video or image data to plural terminals by using a uni-directional point-to-multipoint bearer service. The MBMS is divided into a broadcast mode and a multicast mode. That is, the MBMS is divided into an MBMS broadcast service and an MBMS multicast service.
The MBMS broadcast mode is a service for transmitting multimedia data to every user in a broadcast area. The broadcast area means a broadcast service available area. One or more broadcast areas may exist in one PLMN, one or more broadcast services can be provided in one broadcast area, and one broadcast service can be provided to several broadcast areas.
The MBMS multicast mode is a service for transmitting multimedia data only to a specific user group existing in a multicast area. The multicast area means a multicast service available area. One or more multicast areas can exist in one PLMN, one or more multicast services can be provided in one multicast area, and one multicast service can be provided to several multicast areas.
In the multicast mode, a user is requested to join a multicast group to receive a specific multicast service. At this time, the multicast group refers to a user group that receives the specific multicast service, and joining refers to a behavior of being admitted to the multicast group intending for receiving the specific multicast service.
An MBMS RB, a radio bearer (RB) for the MBMS, serves to transmit user data of one specific MBMS service transferred from the core network to UTRAN to a specific terminal group. The MBMS RB is divided into a point-to-multipoint RB and a point-to-point RB. In order to provide the MBMS service, UTRAN selects one of the two types of MBMS RBs. In order to select the MBMS RB, UTRAN recognizes the number of users of the specific MBMS service existing in one cell. UTRAN internally sets a threshold value, and if the number of users existing in a cell is smaller than the threshold value, UTRAN sets the point-to-point MBMS RB, whereas if the number of users existing in a cell is greater than the threshold value, UTRAN sets the point-to-multipoint MBMS RB.
FIG. 3 shows one example of a process that UTRAN determines a type of the MBMS RB.
In order to transmit specific MBMS service data, UTRAN should set an MBMS RB for a specific MBMS service. For this purpose, UTRAN transmits a group paging signal to terminals which desire to receive the specific MBMS service (step S1) and initiates a paging timer from a transmission time point. The paging timer is expired when it goes beyond a specific value designated by UTRAN.
When a terminal, which has received the group paging signal, desires to receive the MBMS service, the corresponding terminal transmits a paging response signal to UTRAN (step S2). A RRC Connection Setup Request message services as the paging response signal. UTRAN can receive paging response signals from one or more terminals, and receives the paging response signal from terminals until the paging timer is expired.
When the paging timer is expired, UTRAN counts paging response signals received until then, and calculates the number of terminals that desire to receive the MBMS service in the corresponding cell (step S3). UTRAN compares the threshold value and the calculated number of terminals and sets a point-to-multipoint MBMS RB or a point-to-multipoint MBMS RB.
For transmission of the MBMS, UTRAN may operate two types of logical channels for MBMS in each cell: an MBMS control channel (MCCH) and an MBMS traffic channel (MTCH). One MCCH is used to send MBMS-related control information to one or more terminal groups. One MTCH is used to send user data of a specific MBMS service to a terminal group. There is one-to-one correspondence between an MTCH and an MBMS service. The point-to-multipoint MBMS RB for a specific MBMS service consists of the MTCH as well as a corresponding transport channel and a corresponding physical channel.
In the conventional art, common channel configuration information for broadcast or multicast are all transferred to the BCCH. That is, UTRAN transmits the system information block (SIB) including common channel configuration information through the BCCH regardless of a common traffic channel (CTCH) and a common control channel (CCCH). Therefore, if this method is applied to the MBMS, the MTCH and the MCCH configuration information are all to be transmitted through the BCCH.
In this respect, however, in the case of the MBMS service, various services are broadcast or multicast, so that the BCCH has to provide various configurations of MTCH channels. Thus, transmission of all the MTCH and MCCH data through the BCCH causes the current construction of system information transmitted through the BCCH to be changed. In the conventional art, in general, the RB is set up by using a radio bearer setup procedure of the RRC or system information of the RRC.
In the method of using the radio bearer configuration (setup) procedure, UTRAN transmits a radio bearer setup message for transferring RB setup information to one specific terminal, and then the corresponding terminal sets up an RB and transmits a radio bearer setup completion message to UTRAN.
Meanwhile, in the method of using system information, a terminal obtains setup information of a specific RB from system information broadcast by UTRAN and sets up a corresponding RB.
The former method is used when one terminal sets up a specific RB, while the latter method is used when one or more terminals sets up a specific RB. A difference between the two methods is that the former method requires a response message of the terminal while the latter method does not require a response message of the terminal.
Accordingly, in case of setting up the MBMS RB by using the former method is disadvantageous in that radio bearer setup messages in proportion to the number of terminals belong to a terminal group should be transmitted and received, a radio capacity is much taken.
Thus, setup of the MBMS RB in a method similar to the RB setup through the system information is advantageous in terms of the radio capacity, but no process has been substantially defined for setting up the MBMS RB.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.